refactor(): change .md-padding to layout-padding

[marcysutton]

This PR changes all <md-content class="md-padding"> instances in the docs to use [layout-padding] instead.

We discussed making md-content padded by default given the frequency of use in our docs, but I decided against it once I saw how many things it affected. md-content elements are frequently nested, so it undesirably stacked margins against each other.

One outstanding question I have is about the default margin/padding for md-content: currently [layout-padding] uses the baseline grid of 8px, but I see 16px used much more often. IMO, the framework should have one attribute that adjusts its size in SCSS depending on the breakpoint (less margin on small screens). That way, spacing would be consistent across all Angular Material apps. Refer to #1984 and #1322.

Closes #1304

[ThomasBurleson]

@marcysutton - these now look bad:

• http://localhost:8080/#/demo/material.components.bottomSheet
• http://localhost:8080/#/demo/material.components.sidenav
• http://localhost:8080/#/demo/material.components.subheader
• http://localhost:8080/#/demo/material.components.toolbar

Can you resolve as part of this PR?

[marcysutton]

@ThomasBurleson those fixes have all been included. I left layout-padding at 8px because 16px affected a bunch of things, but I do think we need a solution for other larger margins/padding units. That should be a separate discussion we have as a team.